All those fancy parts in your computer need power to work, and you'd do well to invest in a quality power supply (PSU). Again, this is not the place to skimp if you can help it, as buying a no-name unit could endanger your entire PC; if your power source shorts, it could fry your whole system.
Thermaltake, Corsair, Antec, and Cooler Master are some of the most reputable brands when it comes to power supplies, which is why we've gone with Thermaltake's Smart Series 750W. It's got more than enough capacity to power your system, and can even handle dual GPU setups with ease. Plus, it's modular, meaning you can plug in only the power cords you need, which will help keep the insides of your case tidy, and thus improve airflow. Buy it here.
Budget build alternative: Cooler Master eXtreme Power Plus 500W ($37). This is a reliable, name-brand component, and 500W will power our recommended budget build without issue.